The present invention relates to a phase-locked loop circuit that corrects an influence due to a shift in phase during a receiving operation of received signals by a radio data communication terminal in which received signals are equalized by obtaining a transmission line characteristic during a preamble period in a narrow band modulation mode such as the GMSK (Gaussian filtered Minimum Shift Keying) modulation mode. The present invention also relates to a phase locking method.
In many cases, it has been often difficult to use a synthesizer or crystal oscillator with excellent oscillation precision used to generate radio frequency signals. For that reason, a phase-locked loop circuit is normally used in the radio section or baseband section to establish communication under phase error control.
FIG. 4 is a block circuit diagram illustrating a conventional phase-locked loop circuit for BPSK (Binary Phase Shift Keying) modulation. The phased-locked loop circuit consists of a phase comparison circuit 41 for comparing the phase of a received input signal with a remodulated signal, a loop filter 42, a variable frequency oscillator 43, and a remodulation circuit 44 for remodulating demodulated signals.
The BPSK modulation input signal is represented as COS (2.pi.ft+a.pi.) (where f is a carrier frequency and a is a transmission digital signal). The output signal from the variable frequency oscillator 43 is represented as COS (2.pi.ft+a.pi.+.DELTA..PHI.). The signal COS (2.pi.ft+.DELTA..PHI.+b.DELTA.) is obtained by modulating the output signal COS (2.pi.ft+a.pi.+.DELTA..PHI.) with a received digital signal b. The phase comparator 41 compares the two signals and then produces a signal (2.pi.ft+b)-(2.pi.ft+.DELTA..PHI.+a.pi.)(=(b-a).pi.-.DELTA..PHI.). If the received digital signal does not contain an error, a=b. Hence, the phase comparator 41 outputs -.DELTA..PHI. to cancel the phase error .DELTA..PHI. of the variable frequency oscillator 43.
However, the problem is that the conventional phase-locked loop circuit cannot normally receive the modulated signal because the frequency spectrum collapses or becomes asymmetrical in the environment subjected to an influence of a frequency selective multipath fading.